[The Saucepan Man]

Tolkien has Gandalf assert that the dragon was a huge threat to Rivendell, and Gandalf does not say exaggerated or unlikely things

Spot on, Lost One. [img]smilies/smile.gif[/img] Gandalf's quote from Unfinished Tales has, I think, been given before on this thread, but it bears repeating:

You may think that Rivendell was out of [Sauron's] reach, but I did not think so. The state of things in the North was very bad. The Kingdom under the Mountain and the strong Men of Dale were no more. To resist any force that Sauron might send to reagin the northern passes in the mountains and the old lands of Angmar there were only the Dwarves of the Iron Hills, and behind them lay a desolation and a Dragon. The Dragon Sauron might use with terrible effect. Often I said to myself: "I must find some means of dealing with Smaug.

Gandalf obviously thought that, with Smaug in residence at the Lonely Mountain, Sauron would be able to move his force through the northern passes to attack Rivendell. And he was concerned that, if that happened, it might fall. Also, he clearly envisaged that Sauron would be able to enlist Smaug to his cause, to terrible effect. Let's trust Gandalf on this one, shall we? [img]smilies/wink.gif[/img]

As for Smaug's bare patch, I agree that, lucky shots aside, this would be of little avail to those under attack from him without Bilbo's Inside Information. And, in any event, who's to say that, if Smaug fell in with Sauron's forces, it would not be spotted and patched up?